Strains of inbred recombinant mice were examined with respect to (a) their ability to respond to an optimally immunogenic dose of SSS- III, (b) suppressor T cell activity, and (c) amplifier T cell activity. Differences were noted among the strains of mice examined with respect to each of these criteria; however, there was no direct relationship between the effects observed and either allotype or major histocompatibility (H-2) type. These characteristics appear to be mediated by genes which are distinct from the H-2 linked immune response (Ir) genes described by other investigators.